Conflicting evidence has been published that selenium has carcinogenic, anticarcinogenic, or no effect on various species of animals including mice, rats and humans. Our study is designed to determine the relationship between graded (deficient, normal and subtoxic) levels of dietary selenium injestion (0.02, 0.22, and 2.02 ppm Se) and chemical cancer induced in rats by feeding N-2-fluorenyl-acetamide (FAA) at 150, 100 ppm and diethylnitrosamine (DEN) at 80, 40 ppm for predetermined times. Selenium depleted rats will be used as controls. A torula yeast ratin with adequate vitamin E will constitute the basal selenium low diet for supplementation. Selenium and selenium responsive enzymes will be determined on livers and/or tumors and abnormal and normal other body tissues. Rats will be necropsied when moribund or at 200 days. Tissues incuding liver, mammary adeno carcinomas, etc. will be obtained for histological studies for neoplasia. The data obtained will provide evidence as to the effect of selenium on the rate of induction of chemical carcinogenicity (FAA & DEN) in the rat.